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J. Phys. Chem. C 2008, 112, 10176–10186
Stable Anion–Cation Layers on Cu(111) under Reactive Conditions Nguyen T. M. Hai, Klaus Wandelt, and Peter Broekmann* Institute of Physical and Theoretical Chemistry, Bonn UniVersity, Wegelerstrasse 12, 53115 Bonn, Germany ReceiVed: December 2, 2007; ReVised Manuscript ReceiVed: March 29, 2008
Combined voltammetric and in-situ STM studies were employed to gain information about the structure of a sulfate-modified Cu(111) electrode surface exposed to an acidic electrolyte containing a redox-active porphyrin (meso-tetra(N-methyl-4-pyridyl)-porphine, abbreviated as [H2TMPyP]4+). A particular focus of this study lies on the characterization of the interfacial structure under reactive conditions, for example, during an ongoing electron transfer reaction. The oxidized form of [H2TMPyP]4+ cannot be stabilized within the narrow potential window of copper. A two-electron transfer reduction affects the central porphine core even at the anodic limit of the copper potential window close to the onset of the oxidative copper dissolution reaction. This porphyrin-related electron transfer reaction can take place even in the presence of a preadsorbed sulfate/ water coadsorption layer. The latter causes a charge inversion at the metal/anion interface with an excess of negative charges within the sulfate/water layer. Enhanced electrostatic interactions between this sulfate/water coadsorption layer and the cationic porphyrin reactants and reaction products are discussed as the physical origin for the formation of a paired anion–cation layer at the interface that retains its structural integrity even during the ongoing electron transfer reaction, at least on the time scale of the STM experiment. It is the starting sulfate desorption at more negative potentials that causes the loss of lateral order within this paired anion–cation layer. Highly water-soluble cationic porphyrin species coadsorb/codesorb together with the sulfate anions onto/from the Cu(111) electrode. 1. Introduction Charge inversion is a phenomenon of general interest that can play a decisive role in the physicochemical behavior of any charged object exposed to an electrolytic environment.1,2 In general, a macro-ion1,2 can be considered as an object of any topology (extended planar, cylindrical, or spherical, etc.) that reveals a certain excess of charges on its surface. Exposure of this macro-ion, for example, an electrode surface, to an electrolyte will inevitably lead to an accumulation of counterions in the near-surface regime counterbalancing the macro-ion’s charge. Such a screening of charges by the formation of a layer of counter charges is well-known from the early Helmholtz and Gouy-Chapman theory for (metallic) planar electrode surfaces.3,4 While the interfacial electrical double layer as a whole must be electro-neutral, at least under equilibrium conditions, “chargeinversion” means an inversion of the sign of the macro-ion’s charge as “seen” from a finite distance in the outside electrolyte. Charge inversion can be categorized depending on its “physical” or “chemical” origin.1,2 In case of a “physical” charge inversion, solvated ions interact purely via Coulomb forces with each other. Translated to the language of electrochemists, these two origins of charge inversion are related to nonspecific and specific adsorption processes taking place on a metallic electrode surface.4 The latter process occurs in an electrochemical environment in particular when loosely solvated anions chemically interact with a metallic electrode surface. Charge inversion driven by specific anion adsorption is a common phenomenon in surface electrochemistry and known since the early work by Graham on the thermodynamics of mercury electrodes.3 The particular impact of this kind of double layer effect on the * Corresponding author. E-mail:
[email protected], peter.
[email protected].
kinetics of charge transfer reactions is well-known from the early work of Frumkin.5 This article deals with the structural properties of a single crystalline copper electrode exposed to a sulfuric acid electrolyte that additionally contains a cationic redox-active dye (mesotetra(N-methyl-4-pyridyl)-porphine). Charge inversion phenomena are assumed to play a crucial role for the resulting interfacial structures under reactive conditions, in particular when the reactants and reaction products are charged. The adsorption and subsequent lateral ordering of meso-tetra(N-methyl-4-pyridyl)porphine on various iodide-modified electrode surfaces was reported by Itaya’s group.6–8 These previous experiments, however, were restricted to the potential regime where no porphyrin-related electron transfer reaction occurred. Here, we particularly address the interfacial structure under reactive conditions in terms of an ongoing electron transfer reaction. The iodide-modified surfaces used by Itaya’s group were considered as hydrophobic with van der Waals-like forces governing the interaction between the weakly adsorbed cationic porphyrins and the underlying iodide layer.6–9 Here, we use a copper electrode surface as substrate that has to be considered as highly hydrophilic due to the presence of a sulfate/water coadsorption layer on the electrode surface. Its presence on the copper surface causes a charge inversion across the metal/anion interface that can be regarded as the prerequisite for an enhanced electrostatic interaction between this preadsorbed sulfate–water layer and the layer of multivalent cationic porphyrin species on-top. Furthermore, all previous STM studies did not address surface dynamics involving anion adsorption/desorption processes in the presence of the cationic porphyrin layer.6–9 In the present article, we particularly address the impact of sulfate desorption/adsorption processes on the lateral ordering of the paired anion–cation layer.
10.1021/jp711376c CCC: $40.75 2008 American Chemical Society Published on Web 06/14/2008
Stable Anion–Cation Layers on Cu(111) 2. Experimental Section For all solutions, high-purity water (Milli-Q purification system; conductivity
